Real-Time Emission and Exposure Measurements of Multi-walled Carbon Nanotubes during Production, Power Sawing, and Testing of Epoxy-Based Nanocomposites

Abstract

Objectives The use of manufactured nanomaterials is increasing globally. Although multi-walled carbon nanotubes (CNTs) are used in a wide range of applications, only limited data are available on emissions and exposures during CNT composite production. No exposure data using portable aethalometers in the personal breathing zone (PBZ) to monitor occupational exposure to CNTs have yet been published. The aim of this study was to characterize emissions of and exposures to CNTs during CNT composite production, sawing, and shear testing. We also investigated whether real-time aethalometer measurements of equivalent black carbon (eBC) could be used as a proxy filter sampling of elemental carbon (EC). The presence of CNTs as surface contamination in the production facility was monitored since this could contribute to airborne exposure. Methods During CNT composite production in an industrial setting including both chemical and manufacturing laboratories, different work tasks (WTs) were studied with a combination of direct-reading instruments (aethalometer, aerodynamic particle sizer, condensation particle counter) and filter-based methods. Measurements were performed to monitor concentrations in the emission zone (EZ), PBZ, and background zone. The filter samples were analysed for EC and fibre concentration of CNTs using scanning electron microscopy (SEM). Additionally, surfaces in the facility were tape sampled for monitoring of CNT contamination, and analysed with SEM. Results Clear eBC peaks were observed in the PBZ during several WTs, most clearly during open handling of CNT powder. Power sawing emitted the highest particle number concentration in the EZ of both nanoparticles and coarse particles, but no individual airborne CNTs, agglomerates, or aggregates were detected. Airborne CNTs were identified, for example, in a filter sample collected in the PBZ of a worker during mixing of CNT epoxy. The airborne CNT particles were large agglomerates which looked like porous balls in the SEM images. Significant EC exposures were found in the inhalable fraction while all respirable fractions of EC were below detection. The highest inhalable EC concentrations were detected during the composite production. No significant correlation was found between inhalable EC and eBC, most likely due to losses of large EC containing particles in the sampling lines and inside the eBC monitor. In total, 39 tape samples were collected. Surface contamination of CNTs was detected on eight surfaces in the chemical and manufacturing laboratories, mainly in the near-field zone. Elongated CNT-like features were detected in the sawdust after sawing of CNT composite. Conclusions Characterization of a workplace producing CNT composite showed that open handling of the CNT powder during weighing and mixing of CNT powder material generated the highest particle emissions and exposures. The portable direct-reading aethalometer provided time-resolved eBC exposure data with complementary information to time-integrated EC filter samples by linking peak exposures to specific WTs. Based on the results it was not possible to conclude that eBC is a good proxy of EC. Surface contamination of CNTs was detected on several surfaces in the near-field zone in the facility. This contamination could potentially be resuspended into the workplace air, and may cause secondary inhalation exposure.